1. Field of the Invention
The present invention relates to a numerical control apparatus and in particular, the present invention relates to a numerical control apparatus which is capable of specifying shapes of a circular arc and a curved surface which have small curvatures.
2. Description of the Related Art
Conventionally, when a circular arc shape is specified by a machining program, a method for specifying a circular arc shape based on a start point, an end point, and a radius from the center is commonly known. Further, a specifying method in which coordinates of a start point, an end point, and a point on a circular arc are specified is also widely known.
For example, FIG. 10A illustrates an example in which a circular arc shape is specified by a start point, an end point, and a radius from the center. In a circular arc interpolation command “G02 Xp Yp R”, Xp denotes an end point coordinate on a first axis of a plane, Yp denotes an end point coordinate on a second axis, and R denotes a distance from the center. Alternatively, a circular arc shape can be specified as illustrated in FIG. 10B. In a circular arc interpolation command “G02 Xp Yp I J”, Xp denotes an end point coordinate on a first axis of a plane, Yp denotes an end point coordinate on a second axis, I denotes a distance from a start point of the Xp axis to the center of a circular arc, and J denotes a distance from a start point of the Yp axis to the center of the circular arc.
As another prior art technique for specifying a circular arc shape, a method is disclosed in which one point on a perpendicular bisector, which is orthogonal to a specific plane, of a line segment, which is obtained by connecting a start point and an end point, on the plane is selected so as to form a trajectory on a circular arc for avoidance of an interfering object (for example, Japanese Patent Application Laid-Open No. 10-161728).
FIG. 10C illustrates an example in which a circular arc shape is specified by specifying a start point, an end point, and a middle point on a circular arc. In this specifying method, a circular arc shape in a three-dimensional space can be specified. In a circular arc interpolation command illustrated in FIG. 10C, Xxn denotes a coordinate on a first axis in a three-dimensional space, Yyn denotes a coordinate on a second axis, and Zzn denotes a coordinate on a third axis.
However, in the method for specifying a circular arc shape by a start point, an end point, and a radius from the center, the number of command digits exceeds a specification of a numerical control apparatus and therefore, a circular arc shape cannot be specified in a case where a curvature is exceedingly small such as several μm of a width between the start point and the end point and 5 km of a radius from the center as illustrated in FIG. 11, for example. In such a case, a curved surface needs to be specified in a manner to be divided into a minute straight line segments. Therefore, there have been such problems that a CAD/CAM system is separately required and a size of a program is increased.
Further, in the technique described on Japanese Patent Application Laid-Open No. 10-161728, a plane on which a circular arc shape is formed is previously determined, so that it is impossible to form an arbitrary circular arc shape in a three-dimensional space. Further, a point which is specified other than a start point and an end point needs to be set on a perpendicular bisector of a line segment which is obtained by connecting the start point and the end point. Therefore, there is such problem that a load is imposed on an operator who creates a program. Furthermore, in the method described on Japanese Patent Application Laid-Open No. 10-161728, it is impossible to deal with machining of a spherical surface shape and the like.